S
Sean SML Tiling
schluter says 85 rh is fine to crack on with ditra. well thats what i was told.
Anhydrite not dry
Sorry guys. Been pretty busy today with meetings and whatnots. Anyway here I am.
Right in answer to the first question as you already realise the screed ain't dry and there is almost no chance that you will be tiling at the end of the week. At 90mm you would be looking at a natural drying time with a non LL screed of around 140days in good drying conditions. The issue with the drying conditions at the moment of course we are in the worst part of the year and it's flipping wet so that will go up. Additionally there is some evidence to suggest that the addition of the LL additives used in some screeds impedes the drying rates so will slow it down further.
You have 2basic options. Dry it out further ideally using the heating or put the whole thing at risk.... The latter is a bit tongue in cheek as I know you ain't gonna do that.
Running up the heating again may work but at that sort of moisture level you are likely to need a full commissioning cycle again so you are looking at least three weeks and at that depth and with that moisture level possibly a little longer.
90mm is just too deep in my opinion and the use of non LL would have been far better IMO as removing the Laitance to provide a nice open surface texture has been shown to aid drying but there you go.
I think,I am right in saying gypfix can not be used at levels above 75% these days although it used to be 85%.
According to suchluter, ditra can be stuck down to anhydrite at 2% moisture but the million dollar question is "what with"?? As I am unaware of any adhesives that will work at that higher level.
The danger of entrapment of the moisture is more to do with damaging and weakening the interface between primers and screed or adhesive and screed rather than weakening the screed itself. There are combinations of things than can club together to cause the failure if moisture is present but the most common is emulsification of the primer. Another potential issue tp you have with the LL screeds is the difficulty it creates in getting the primer into the surface of the screed. I tend to suggest primerless or at least heavily diluted primers for these types of screed.
The other thing to double check is that there is actually an operative DPM between the screed and the substrate. This would be a 1200g polythene membrane underneath any insulation but on top of any concrete or beam and block.
Right in answer to the first question as you already realise the screed ain't dry and there is almost no chance that you will be tiling at the end of the week. At 90mm you would be looking at a natural drying time with a non LL screed of around 140days in good drying conditions. The issue with the drying conditions at the moment of course we are in the worst part of the year and it's flipping wet so that will go up. Additionally there is some evidence to suggest that the addition of the LL additives used in some screeds impedes the drying rates so will slow it down further.
You have 2basic options. Dry it out further ideally using the heating or put the whole thing at risk.... The latter is a bit tongue in cheek as I know you ain't gonna do that.
Running up the heating again may work but at that sort of moisture level you are likely to need a full commissioning cycle again so you are looking at least three weeks and at that depth and with that moisture level possibly a little longer.
90mm is just too deep in my opinion and the use of non LL would have been far better IMO as removing the Laitance to provide a nice open surface texture has been shown to aid drying but there you go.
I think,I am right in saying gypfix can not be used at levels above 75% these days although it used to be 85%.
According to suchluter, ditra can be stuck down to anhydrite at 2% moisture but the million dollar question is "what with"?? As I am unaware of any adhesives that will work at that higher level.
The danger of entrapment of the moisture is more to do with damaging and weakening the interface between primers and screed or adhesive and screed rather than weakening the screed itself. There are combinations of things than can club together to cause the failure if moisture is present but the most common is emulsification of the primer. Another potential issue tp you have with the LL screeds is the difficulty it creates in getting the primer into the surface of the screed. I tend to suggest primerless or at least heavily diluted primers for these types of screed.
The other thing to double check is that there is actually an operative DPM between the screed and the substrate. This would be a 1200g polythene membrane underneath any insulation but on top of any concrete or beam and block.
A lot of builders don't use a polythene membrane. Especially when using polypipe ( egg box plates) I hate them.
If ths is done on block and beam, ie block- kingspan- egg box- pipe then screed. You can have the scenario where you have the moisture from the blocks and anything that is under them to concede with!
If ths is done on block and beam, ie block- kingspan- egg box- pipe then screed. You can have the scenario where you have the moisture from the blocks and anything that is under them to concede with!
K
kaharrison9
Many thanks and a wonderful response.
I will need to look into the RH maximum tomorrow as the screed company and the screed layer both mentioned 85 rh max also I myself thought it was that but the Gypfix technician mentioned 75 and 80 absolute max.
The screed companies logo is on the gypfix bag so both companies do work together but are giving out different figures.
I do wonder if it is know 75 RH what has happened for it to have been reduced.
Have digested your info on drying on LL screeds and it is by far the thickest screed I have worked on so understandable how long this is going to take to dry.
Interesting point about checking there is a DPM as well.
I think a full commissioning with better ventilation over a 3-4 week period is the only option I can offer.
The customer has told all trades he would like to be in for April so tomorrow could be interesting!
I will need to look into the RH maximum tomorrow as the screed company and the screed layer both mentioned 85 rh max also I myself thought it was that but the Gypfix technician mentioned 75 and 80 absolute max.
The screed companies logo is on the gypfix bag so both companies do work together but are giving out different figures.
I do wonder if it is know 75 RH what has happened for it to have been reduced.
Have digested your info on drying on LL screeds and it is by far the thickest screed I have worked on so understandable how long this is going to take to dry.
Interesting point about checking there is a DPM as well.
I think a full commissioning with better ventilation over a 3-4 week period is the only option I can offer.
The customer has told all trades he would like to be in for April so tomorrow could be interesting!
that was a marketing thing I set up when I was at gyvlon...
the reason it was reduced from 85 to 75 was simply to reduce risks of moisture entrapment under large format tiles and to take account of the possible difference between a builders 85% and a real 85% rh.
W
White Room
Whats an LL screed or have I missed somethink....
K
kaharrison9
So in theory could 85 rh be accepted if ditra is used as any moisture entrapment under a large format marble tile would not interfere with the screed as the ditra would form a physical barrier and I assume the moisture would then naturally come up and evaporate through the stone before it has been sealed.
K
kaharrison9
LL = Low laitance screed
W
White Room
Cheers matey....
No mate. If you use ditra over a wet screed the last thing you want is the moisture coming up through the stone as it will stain it. You would need to tape the ditra joints and let the mat do it's thing. In theory 85% would be fine but you need to find what you can stick it down with. Possibly gypfix, Anhyfix or one of the other stable of gypsum adhesives.
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